1. Field of the Invention
The present invention relates to a signal processing circuit for a tuner, and in particular to a signal processing circuit for a tuner which applies signal processing based on a frequency variation when a carrier frequency of an input intermediate frequency (“IF”) signal is varied.
2. Description of the Related Art
An AM (amplitude modulation) system is used for transmitting a signal to be transmitted (modulating signal) on an amplitude of a signal (carrier) having a frequency which can be radiated from a broadcasting station and is primarily used in medium frequency radio broadcasting (526.5 kHz–1606.5 kHz). Radio waves in a medium frequency radio broadcasting frequency band have a characteristic that a wide service area can be secured and it is possible to provide a stable service for a moving body such as a car, because these radio waves are not only transmitted as ground wave but also transmit as space waves reflected by the ionosphere (E layer) around a height of 100 km above the ground, particularly during night time.
A superheterodyne detection method is typically used in a receiver device for receiving an AM signal. In a superheterodyne detection method, a signal from a broadcasting station and a signal from an oscillator circuit (local oscillator circuit) provided in the receiver device are synthesized, a beat between the two synthesized signals is detected and converted to an intermediate frequency signal, and the intermediate frequency signal is amplified and demodulated. The superheterodyne detection method has characteristics that a high amplification gain can be easily obtained and crosstalk can be easily prevented. In addition, it is necessary to provide a band pass filter for allowing only a desired broadcasting frequency in the receiver circuit for receiving an AM signal. Because it is very difficult to continuously change the center frequency of a band pass filter without changing the band characteristics of the filter, a method is employed in which the local oscillation frequency is changed to allow only a signal of an intermediate frequency converted to a predetermined frequency to pass through.
In a receiver device for receiving an AM signal, the level of a detected signal which is an audio signal to be output varies, or crosstalk tends to occur depending on the intensity of an electric field of a received broadcasting wave input from an antenna.
As a method for reducing the occurrence of crosstalk, a method is proposed for changing the sensitivity of an automatic gain control (AGC) circuit for amplifying the received RF signal based on the intensity of electric field of a received broadcasting wave (for example, in Japanese Patent Laid-Open Publication No. Hei 7-22975).
With advances in digital signal processing techniques, a signal processing circuit for a tuner has been realized in which an intermediate frequency signal is converted into a digital signal by an analog-to-digital converter (ADC), the intermediate frequency signal is detected through digital signal processing, and a demodulated signal is obtained.
A carrier frequency of an intermediate frequency signal which is a signal from an AM tuner is typically 450 kHz. The intermediate frequency signal is obtained by tuning a frequency of a local oscillator in the tuner based on a frequency of a broadcast wave of a desired station. In the local oscillator for obtaining an intermediate frequency of 450 kHz from a carrier of a broadcasting wave of a medium frequency band, typically, a quartz resonator of 10.25 MHz is used. This frequency is determined by the fact that the frequency spacing in medium frequency broadcasting is 9 kHz.
Digital audio devices other than the tuner such as, for example, a compact disk player (CDP) and a minidisk player (MDP), on the other hand, typically operate based on a quartz oscillator of 16.9344 MHz. Because of this, it is possible to use the clock frequency of a tuner DSP (digital signal processor) common to other digital audio devices so that the development cost of a circuit and software for digital signal processing can be reduced.
In addition, a signal processing circuit for a tuner may also receive digital input from devices other than a tuner such as, for example, a digital audio device such as a CDP and MDP and apply signal processing such as sound quality correction within a single chip. In this configuration, because provision of two types of different clock frequencies would increase the cost, the signal processing circuit for the tuner also is required to operate with a common reference clock frequency.
Therefore, in general, a signal processing circuit for a tuner and the tuner itself operate based on separate and independent frequencies of quartz resonators. In an actual product, these frequencies may be different because of individual differences of the quartz resonator, environmental temperature, etc.
In a detection process in a signal processing circuit for a tuner, the variation of carrier frequency of an input intermediate frequency signal with respect to a clock frequency of the signal processing circuit for a tuner can cause increase in degradation of SN ratio of the demodulated signal and degradation of adjacent interference detection precision.